The threat to supplies bound for China, a major importer of New Zealand baby formula, was not initially disclosed publicly by New Zealand authorities, who tightened access to it in domestic shops.

They also launched a massive investigation into the threat to New Zealand’s dairy sector, the country’s biggest export earner.

Police interviewed 2,600 people, eventually arresting Kerr in 2015 after DNA evidence linked him to the letters.

Prosecutors said he owned the rights to a pesticide that was a market rival to 1080 and would gain financially if a ban was implemented.

A high court hearing was told he was under financial pressure and suffering mental health problems following his wife’s death when the threats were made.

He told police after his arrest that reference to the Chinese market was just to “add some impact”, the hearing was told.

Judge Geoffrey Venning on Wednesday accepted Kerr never meant to carry out the poison threat but said it was still one of the most serious cases of blackmail to come before the courts.

He said the threat could have jeopardised trade relations and had cost affected parties NZ$32m, including the police investigation and security measures implemented by Fonterra.

Noting that the maximum term for blackmail is 14 years, Venning sentenced Kerr to eight and a half years’ jail.

Fonterra safety manager Maury Leyland said the threat had a very real, emotional and financial cost on Fonterra’s staff and business as well as on ordinary people. “It is hard to imagine a worse threat to children and families,” she said in a victim impact statement.